Corifollitropin alfa reaches maximum concentrations (Cmax), between 25 and 45 h after injection , a time interval which is significantly shorter as compared to treatment with rFSH. The investigators hypothesized that this rapid increase in the serum FSH concentration may result in a significantly higher exposure of the small antral follicles to constant high levels of FSH during the early follicular phase, securing not only the recruitment of the follicles, but also the continued growth.